RATIONALE: Drugs used in chemotherapy use different ways to stop cancer cells from dividing so they stop growing or die. Combining chemotherapy with dexamethasone may be an effective treatment for multiple myeloma.

PURPOSE: Phase II trial to study the effectiveness of combining isotretinoin and dexamethasone in treating patients who have multiple myeloma.

OBJECTIVES: I. Determine the response rate and duration of response of patients with multiple myeloma treated with dexamethasone and isotretinoin. II. Determine the toxicity of this regimen in these patients. III. Correlate the changes in serum interleukin (IL)-6, IL-6R, and C-reactive and IL-6R expression on plasma cells in the bone marrow with response in patients treated with this regimen.

OUTLINE: Patients receive oral dexamethasone on days 1-4, 9-12, and 17-20 and oral isotretinoin daily. Treatment repeats every 5 weeks for at least 2 courses in the absence of disease progression, insufficient response, or unacceptable toxicity. Patients achieving adequate response continue treatment for 2 courses after achieving a plateau of monoclonal protein, for a minimum of 6 courses total.

PROJECTED ACCRUAL: A total of 18-36 patients will be accrued for this study within 3 years.

• Drug: dexamethasone
• Drug: isotretinoin

DISEASE CHARACTERISTICS: Histologically confirmed multiple myeloma Elevated monoclonal protein in serum and/or urine

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-2 Life expectancy: Not specified Hematopoietic: WBC at least 3,000/mm3* Platelet count at least 75,000/mm3* *Unless secondary to multiple myeloma involvement of the bone marrow Hepatic: Bilirubin less than 1.5 mg/dL Transaminase less than 2 times normal Renal: Creatinine less than 2.0 mg/dL No overt renal insufficiency Cardiovascular: No congestive heart failure No myocardial infarction within the past 6 months No significant arrhythmia or poorly controlled hypertension Pulmonary: No severe pulmonary disease Other: Triglycerides normal No other severe medical illness No active peptic ulcer disease No brittle insulin-dependent diabetes No severe depression or other psychiatric illness No active infection No history of severe ethanol or drug abuse Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior chemotherapy Endocrine therapy: Not specified Radiotherapy: Prior palliative radiotherapy allowed Surgery: At least 3 weeks since prior major surgery